In refrigeration machines of the vapor compression type, wherein the compressor and the compressor drive motor and transmission are hermetically sealed, liquid refrigerant is used to cool the motor driving the compressor. The oil used to lubricate the motor, the transmission and the compressor comes into contact with the refrigerant and absorbs a portion of the refrigerant. Excess oil is collected in a sump within the hermetically-sealed housing and an oil pump is used to recirculate the oil to the bearings and transmission requiring lubrication. Since the unit is hermetically sealed, the atmosphere within the housing is vaporized refrigerant. In order to reduce windage losses in the operation of the transmission it is desirable to keep the pressure of vaporized refrigerant within the housing as low as possible. This is normally accomplished by venting the transmission housing to the suction line of the compressor through a pressure equalization line. Because of the low pressure in the transmission, relatively small amounts of refrigerant are absorbed by the oil. However, during periods when the refrigeration machine is shut down, the pressure in the transmission housing increases and the amount of refrigerant absorbed by the oil increases. Upon start-up of the compressor, the pressure in the suction line to the compressor immediately drops causing a corresponding drop of pressure in the transmission housing. As a result of the drop in pressure in the transmission housing, the refrigerant absorbed in the oil tends to boil out of the oil as a vapor. If the boiling of the refrigerant is vigorous, oil foaming or cavitation of the oil pump results. Oil pump cavitation produces a loss of oil pressure in the system and, with safety controls protecting the equipment from damage from loss of oil pressure, the refrigeration machine is usually shut down. In order to avoid shutdown and recycling of the equipment, it is desirable to prevent foaming of the oil or oil pump cavitation. On prior commercial equipment, pressure sensing devices have been provided to sense oil pressure and, in the event of low oil pressure, to close inlet guide vanes in the compressor suction line thus reducing the rate of pressure drop in the suction line and in the transmission housing. However, when an oil pressure drop is sensed, oil pump cavitation has already occurred. The present invention is intended to both reduce the amount of boiling of refrigerant upon start-up of the compressor and to anticipate a condition which would create oil pump cavitation so that corrective steps may be taken prior to a drop in oil pressure.